Color cathode ray tubes are, as shown in FIG. 4, have a bulb 8 formed by uniting a face plate 4 in which a fluorescent layer 2 is formed on the inside surface of a funnel 6. At the inside of a neck portion of bulb 8, there is received an electron gun 10 which emits R,G,B electron beams 12. The beams pass through apertures of a shadow mask 14 and then strike fluorescent layer 2 to form a picture element on a screen. A deflection yoke 16 disposed on the outside surface of the funnel 6 deflects the beams to form the picture on the screen.
The inline type electron gun for such color cathode ray tubes has advantages of being easily manufactured due to the simple structure in which R,G,B electron beams are transversely arranged in a line and omitting vertical dynamic convergence. However, this gun has the disadvantage of having serious spherical aberration of a focus lens for focusing the electron beams.
To reduce spherical aberration, it is well known that greater spacing between a pair of electrodes forming the focus lens creates an expanded focus lens. In this case, the greater the spacing between the pair of electrodes forming the focus lens, the smaller the spherical aberration is. However, when the spacing therebetween is excessively great, static electric charge generated from the periphery of the neck portion has influence on the focus lens to cause the electron beams misconvergence, which severely restricts the expanded focus lens of the inline type electron gun.
Thus, attempts at forming the expanded focus lens have been continued. One of them is described in U.S. Pat. No. 4,370,592 "Color Picture Tube having an Improved Inline electron Gun with an Expanded Focus Lens", issued to Richard H. Hughes on Jan. 25, 1983. FIG. 9 shows the main focus lens structure of the inline type electron gun provided in the U.S. Pat. No. 4,370,592. The electron gun of such method has R,G,B apertures 18.sub.R, 18.sub.G, 18.sub.B, 20.sub.R, 20.sub.G, 20.sub.B which are included in a pair of electrodes 18,20, facing each other and has a horizontal lens 22,24 which is formed by deep drawing at a predetermined depth from the opposing side of the two electrodes 18,20 at their peripheries, thereby, in practice, forming the expanded main focus lens without the influence of external static charge. The width of both horizontal lenses 22,24 is, in practice, determined by the lateral width D of the third electrode 18 and by the lateral width E of the fourth electrode 20. While the lateral width D,E is severely restricted by the inside diameter of the neck portion of the color cathode ray tube (CRT), in a practical manufacturing process, it is much restricted by the distance A between the axes of each aperture. That is, while the horizontal lens 22,24 can be used in a SS(small separation) type of 5.08 mm or in a MS(middle separation) type of 5.5-5.6 mm, it can not be used in a LS(large separation) type of more than 6.6 mm.
The reason for not applying the horizontal lens to the LS type will be described with reference to FIG. 6, which is a plan view of the fourth electrode 20. Each R,G,B electron beam E.sub.R,E.sub.G,E.sub.B which pass through each aperture 20.sub.R,20.sub.G,20.sub.B thereof is positioned at the center thereof. However, when the distance A between the axes of each aperture is more than 6.6 mm, the respective distance from the two side apertures 20.sub.R,20.sub.B to the two ends of the horizontal lens 24 becomes different from each other, whereby each horizontal focusing voltage between the central aperture the two side apertures is different.
The reason for the horizontal focusing voltage difference above is that the side apertures 20.sub.R, 20.sub.B are nearer the electrode 20 than the central aperture 20.sub.G, whereby the side apertures have higher potential than the center aperture. For the same reason as above, the two side apertures 20.sub.R,20.sub.B have a partial or local difference between the horizontal and vertical focusing voltage, whereby the picture element formed on the screen is distorted.
To solve such problems caused by the expanded focus lens as described above, the multi-focusing type electron gun of a uni-uni-bipotential lens arrangement as shown in FIG. 7 has been used in practice. Such electron gun is formed such that a cathode K, an electrode assembly formed by first to eighth electrodes 26 to 40, and a shield cup 42 are successively arranged. A screen voltage Vs of 0-1 Kv is applied to the second, fourth and sixth electrode 28,32,36, a focus voltage Vf of 0-10 Kv is applied to the third, fifth and seventh electrodes 30,34,38, a voltage of 0--100 Kv is applied to the first electrode 26 and 0-30 Kv high voltage is applied to the eighth electrode 40 and the shield cup 42. Accordingly, the third, fourth and fifth electrode 30,32,34 form a unipotential lens SL1, the fifth, sixth and seventh electrode 34,36,38 form a second unipotential lens SL2 and the seventh and eighth 38,40 form a bipotential lens ML.
The uni-uni-bipotential type electron gun can optically be explained with reference to FIG. 8, wherein a solid line indicates that the first emitting angle .theta..sub.1 of the electron beam becomes small when the first unipotential lens SL1 is larger than the second unipotential lens SL2. As a result, since the electron beam which passes through the first lens centrally enters the axis of the second lens, the second emitting angle .theta..sub.2 becomes relatively small for the beam to have a comparatively small inside diameter r. By contrary, a dotted line reveals that, since the first emitting angle .theta..sub.1, becomes large when the first lens SL1' is formed less weakly than the second lens SL2', the beam enters the surroundings of the second unipotential lens SL2. Accordingly, even when the second emitting angle .theta..sub.2 ' of the beam becomes small, the beam has a comparatively large inside diameter r'. Further, the diameter of the two unipotential lenses SL1,SL2 is determined by the spacing between the opposing electrodes so that, when the second lens is expanded, static charge generated from the neck portion has serious influence thereon to reduce picture quality.
Thus, the first unipotential lens of a conventional multi-focusing type electron gun is formed with a larger diameter than the second lens so that the beam has a small inside diameter. However, even in such a gun, problems arise when distribution of equipotential formed in each aperture of the sixth electrode 36, which is the center of the second unipotential lens SL2 is examined.
FIG. 9 explains distribution of equipotential of the second unipotential lens SL2 formed at the aperture inside of the sixth electrode 36 of the multi-focusing electron gun of FIG. 7. Bordering each aperture inside the sixth electrode 36 to which the relatively low voltage is applied, equipotential lines are formed of a bobbin type in which the middle part is depressed, so a gap L formed at a symmetrical point becomes very narrow to hinder keeping the voltage difference uniform.
Accordingly, picture quality of color cathode ray tubes can be much improved when the second unipotential lens SL2 is not influenced by static charge generated from the neck and voltage difference thereof is kept uniform.